To improve deterioration in communication quality due to the recent rapid increase in mobile traffic and to achieve higher-speed communication, 3GPP Long Term Evolution (LTE) specifies a carrier aggregation (CA) function to allow a radio base station (eNode B (eNB)) and a radio terminal (User Equipment (UE)) to communicate with each other using a plurality of cells. The cells which can be used by the UE in the CA are limited to cells of one eNB (i.e., cells that are served or managed by the eNB). The cells used by the UE in the CA are classified into a primary cell (PCell) that is already used as a serving cell when the CA is started and a secondary cell(s) (SCell(s)) that is used additionally or subordinately. In the PCell, Non Access Stratum (NAS) mobility information (NAS mobility information) and security information (security input) is sent and received during radio connection (re)-establishment (RRC Connection Establishment, RRC Connection Re-establishment) (see Section 7.5 in Non-Patent Literature 1).
In the CA, SCell configuration information transmitted from the eNB to the UE includes SCell radio resource configuration information common to UEs (RadioResourceConfigCommonSCell) and SCell radio resource configuration information dedicated to a specific UE (RadioResourceConfigDedicatedSCell). The latter information mainly indicates a dedicated configuration (PhysicalConfigDedicated) for a physical layer. When cells (carriers) having different transmission timings (Timing Advance: TA) are aggregated in an uplink, configuration information (MAC-MainConfigSCell) about a Medium Access Control (MAC) sublayer is also transmitted from the eNB to the UE. However, the configuration information about the MAC sublayer includes only an STAG-Id, which is an index of TA Group (TAG) representing a set of cells included in the same TA (see Section 5.3.10.4 in Non-Patent Literature 2). The other configurations for the MAC sublayer in the SCell are the same as those in the PCell.
One of the ongoing study items in the LTE standardization related mainly to a Heterogeneous Network (HetNet) environment is dual connectivity in which the UE performs communication using a plurality of cells of a plurality of eNBs (see Non Patent-Literature 3). Dual connectivity is a process to allow an UE to perform communication simultaneously using both radio resources (i.e., cells or carriers) provided (or managed) by a main base station (master base station, Master eNB (MeNB)) and a sub base station (secondary base station, Secondary eNB (SeNB)). Dual connectivity enables inter-eNB CA in which the UE aggregates a plurality of cells managed by different eNBs. Since the UE aggregates radio resources managed by different nodes, dual connectivity is also called “inter-node radio resource aggregation”. The MeNB is connected to the SeNB through an inter-base-station interface called Xn. The MeNB maintains, for the UE in dual connectivity, the connection (S1-MME) to a mobility management apparatus (Mobility Management Entity (MME)) in a core network (Evolved Packet Core (EPC)). Accordingly, the MeNB can be called a mobility management point (or mobility anchor) of the UE. For example, the MeNB is a Macro eNB, and the SeNB is a Pico eNB or Low Power Node (LPN).
Further, in dual connectivity, a bearer split for splitting a network bearer (EPS bearer) over the MeNB and the SeNB has been studied. The term “network bearer (EPS Bearer)” used in this specification means a virtual connection that is configured between a UE and an endpoint (i.e., Packet Data Network Gateway (P-GW)) in a core network (EPC) for each service provided to the UE. In an alternative of the bearer split, for example, both a radio bearer (RB) in a cell of the MeNB and a radio bearer in a cell of the SeNB are mapped to one network bearer. The radio bearer (RB) described herein refers mainly to a data radio bearer (DRB). The bearer split will contribute to a further improvement in user throughput.